This invention relates to a coating composition and, more particularly, to a coating composition suitable for providing polyolefin formed products, such as polyolefin sheets, with a uniform, matte coating which exhibits good adhesion to the products, good abrasion resistance, and good chemical resistance. The invention relates also to polyolefin products coated with such coating composition.
Soft polyvinyl chloride is frequently used for sheets, from which, for example, interior furniture of automobiles is formed. However, soft polyvinyl chloride products are disadvantageously subject to discoloration and tend to become brittle due to a large amount of plasticizer used therein for providing softness to the products.
Recently, polyolefin, e.g. olefin thermoplastic elastomer, is frequently used in place of soft polyvinyl chloride because polyolefin itself is soft and, therefore, need not be softened by use of any plasticizer. In addition, polyolefin has a smaller specific gravity than soft polyvinyl chloride, which makes it possible to provide lighter products than soft polyvinyl chloride. Furthermore, polyolefin is recyclable.
An olefin thermoplastic elastomer used for forming, for example, interior furniture of automobiles, includes an ethylene-propylene copolymer, or an ethylene-propylene-diene terpolymer consisting of an ethylene-propylene copolymer and a third component having a partial bridge structure, with an antistatic agent, a thermostabilizer, an ultraviolet absorbing agent, a pigment etc. added to it.
Generally, it is difficult to apply a coating over the surface of a product of such olefin thermoplastic elastomer, and if a coating can be provided, the coating will have poor physical properties, such as poor adhesion and poor abrasion resistance. This is because the surface of polyolefin products is inactive.
In prior art, polyolefin products are provided with a coating by first activating the surface of a product with corona discharge, then, coating the activated surface with a chlorinated polyolefin primer or a two-pack curing primer including, for example, polyester and polyisocyanate, or polyurethane and polyisocyanate, and finally providing a top coating. Such a method is disclosed in Japanese Unexamined Patent Publication Nos. SHO 63-272547 A, HEI 7-247381 A and HEI 7-247382 A.
Recently, people tend toward higher class interior furniture for automobiles, and, therefore, furniture formed simply of olefin thermoplastic elastomer is hardly seen. For example, trim panels for automobiles are frequently formed of polyolefin sheet over which fabric or the like is bonded by an adhesive. Accordingly, surface treatment of polyolefin products must provide products with resistance against solvent in the adhesive used to bond fabric to polyolefin products and creep resistance against temperature within automobiles.
Accordingly, two-pack curing type coating compositions composed of polyester and polyisocyanate or polyurethane and polyisocyanate are used in many cases. However, recently, the use of polyisocyanate curing agents has raised problems in human health and safety. Further, two-pack curing type coating compositions in which polyisocyanate is used as a curing agent have a disadvantageously short pot life because the polyisocyanate curing agent, when it is mixed, increases the viscosity of the coating compositions.
According to the present invention, a coating composition for treating the surface of a polyolefin resin product is provided. The coating composition includes a curing agent of polycarbodiimide compound. The use of polycarbodiimide compound as the curing agent can solve the above-discussed various problems, and can provide a matte, uniform coating which exhibits good adhesion to a polyolefin product, good heat resistance, and good solvent resistance.
The coating composition according to the present invention is composed of 100 parts by weight in solid content of polymer solution or water dispersion essentially consisting of a polyurethane resin or a polyester resin, and from 2 to 100 parts by weight of a polycarbodiimide compound. The polymer solution or water dispersion is referred sometimes to as main component.
The polyurethane or polyester resin may contain a carboxyl group in an amount not greater than unity (1) in terms of acid value.
According to the present invention, a polyolefin product, e.g. a polyolefin sheet is coated with the coating composition with or without a carboxyl group.
A polyolefin product may be coated with primary and secondary coatings. The primary coating is formed by applying the above-described coating composition with or without a carboxyl group, over the surface of the product, and the secondary coating is formed by applying a polyurethane solution over the primary coating.
The polyurethane resin in the secondary coating may also contain carboxyl group.
Before applying the coating composition, the surface of the polyolefin product to be coated is preferably treated with corona discharge, which increases adhesion of the coating to the product surface.
The polycarbodiimide compound used as a curing agent in the present invention is a compound which includes a highly reactive carbodiimide group expressed as xe2x80x94Nxe2x95x90Cxe2x95x90Nxe2x80x94 in its main chain. The carbodiimide group reacts with an active hydrogen group, e.g. a carboxyl group, an amino group or an hydroxyl group, to thereby provide a cross-linked structure. In particular, the polycarbodiimide compound can react with the carboxyl group at normal temperature. When a polyurethane or polyester resin containing no carboxyl group is used for the main component of the coating composition, the adhesion of the resulting coating to a polyolefin product can be provided by treating the polyolefin product with corona discharge prior to the application of the coating composition. By corona discharge, an active group, e.g. a hydroxyl group or a carboxyl group, may be produced on the surface of the polyolefin product. The carbodiimide group in the polycarbodiimide compound in the coating composition reacts with such hydroxyl group or carboxyl group or with an amino group or a hydroxyl group or water in the main component, which makes the resulting coating adhere to the surface of the polyolefin product. The coating composition composed of the main component with no carboxyl group and a polycarbodiimide compound has no pot life problem.
On the other hand, by introducing a carboxyl group into the above-described main component of the coating composition, or by using a polyurethane or polyester resin having a carboxyl group, the polyurethane or polyester resin can rapidly crosslink with the carbodiimide group. This results in a coating having good solvent resistance and good abrasion resistance. However, the resulting coating composition with a carboxyl group is more or less unstable and short in pot life.
Knowing this-fact, the inventors have found that the amount of the carboxyl group in the. main component is preferably not greater than unity (1) in terms of acid value. A coating composition composed of the main component containing a carboxyl group in this: amount and a polycarbodiimide compound is applied over the surface of a polyolefin product to form a coating. Further, this coating may be used as a primary coating, over which a secondary coating of polyurethane is disposed. The polycarbodiimide compound in the primary coating moves into the secondary coating and reacts with a carboxyl group in the secondary coating. The resulting composite coating exhibits not only good adhesion to the polyolefin product but also good solvent resistance and good abrasion resistance.
A polycarbodiimide compound commercially available from Nisshinbo Industries, Inc., Japan or from Union Carbide, Inc., U.S.A. may be used as the polycarbodiimide compound in the present invention. As for the amount of the polycarbodiimide compound, when the polycarbodiimide resin in an amount of less than 2 parts by weight to 100 parts by weight of the main component in solid content is used, the resulting coating exhibits poor adhesion to a polyolefin product and insufficient solvent resistance. If the amount of the polycarbodiimide compound exceeds 100 parts by weight, the resulting coating exhibits satisfactory adhesion, but poor solvent resistance. Accordingly, the amount of the polycarbodiimide compound is from 2 to 100 parts by weight relative to 100 parts by weight of the main component in terms of solid content.
The polymer resin solution or water dispersion mainly composed of polyurethane or polyester resin may be provided by various combinations of materials. The polyurethane or polyester resin of the main component may be in the form of a blend with another polymer resin, or may be in the form of a copolymer or graft polymer with a monomer forming another polymer resin. For example, the polyurethane or polyester resin of the main component may be blended with chlorinated polyolefin resin, silicone resin, acrylic resin, epoxy resin, cellulose resin, nitrile rubber or other resin, depending on the use of the resulting coating composition. The resin of the main component may be in the form of a graft polymer with acrylic monomer or copolymer with reactive silicone resin.
The polyurethane resin solution of the main component can be manufactured by, for example, heating a polyol having a terminal hydroxyl group and having an average molecular weight of from about 500 to 5,000, in an organic solvent together with organic diisocyanate, and a chain extender. Such polyol may be polyester diol, polyether diol, lactone diol, or polycarbonate diol.
To prepare a solution of polyurethane resin with a carboxyl group, to be used as the main component, dimethylol propionic acid, which is a diglycolic acid, is preferably used as the chain extender.
The polyester diol useable in preparing the polyurethane resin for the coating composition of the present invention can be prepared by condensating a dicarboxylic acid with a diol. The dicarboxylic acid may be adipic acid, sebacic acid, terephthalic acid, or isophthalic acid, and the diol may be ethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, or neopentyl glycol.
The polyether diol may be polyethylene glycol, polytetramethylene glycol, or their copolymer ethers. The lactone diol may be polylactone diol prepared by ring-opening polymerization of xcex5-caprolactone. An example of the polycarbonate diol is polyhexamethylene carbonate diol.
The organic diisocyanate useable in the present invention may be 2,4 or 2,6-tolylenediisocyanate (TDI), 4,4xe2x80x2-diphenylmethane diisocyanate (MDI), hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), or 4,4xe2x80x2-dicyclohexylmethane diisocyanate (H2MDI).
The chain extender may be diglycolic acid, or a diamine, e.g. ethylene diamine, isophorone diamine, 4,4xe2x80x2-diaminodicyclohexylmethane, hydrazine, or piperazine.
The organic solvent useable in the present invention may be dimethyl formamide (DMF), dimethyl acetamide, methyl ethyl ketone (MEK), acetone, toluene, xylene, dioxane, tetrahydrofuran, ethyl acetate, butyl acetate, isopropyl alcohol, or N-methyl pyrrolidone. These solvent may be used solely or in combination.
The polyester resin used as the main component of the coating composition of the present invention may the ones exemplified above as the materials for preparing the polyurethane resin, but its average molecular weight is preferably 10,000 or more.